Saturday, September 01, 2007

Poincare was correct about the aether

Steve Sailor and John Hawks comment on the top scientists in history. It is good to see credit to 20th century geniuses like R. A. Fisher (invented much of modern statistics), Leo Szilárd (invented the A-bomb by first conceiving the idea of a nuclear chain reaction), John von Neumann (helped create quantum mechanics and electronic computers), and Claude Shannon (invented information theory).

Poincare surely belongs on the list also, for creating the theory of special relativity, but a commenter repeats this canard:

Poincare formulated what is called the principle of relativity, but I don't think he devised special relativity before Einstein. (But maybe you know something I haven't heard about.) Lorentz came damn close and predicted many of the effects of special relativity. Unfortunately his theory did not discard the idea of ether and didn't quite pan out experimentally.

It has become popular to credit Henri Poincaré with the discovery of the theory of Special Relativity, but sadly Poincaré got many of the right answers for all the wrong reasons. He even came up with a version of E = mc2! In 1904 Poincaré had gone as far as to enunciate the "principle of relativity" in which "The laws of physical phenomena must be the same, whether for a fixed observer, as also for one dragged in a motion of uniform translation, so that we do not and cannot have any means to discern whether or not we are dragged in a such motion." In 1905 Poincaré coined the term "Lorentz Transformation" for the equation that explained the null result of the Michelson Morley experiment. Although Poincaré derived equations to explain the null result of the Michelson Morley experiment, his assumptions were still based upon an aether. It remained for Einstein to show that an aether was unnecessary, a conceptual leap that thwarts many students even today.

These last two sentences are completely wrong. Let's look at what Poincare actually said about the aether. In 1900, he said:

Our ether, does it really exist? I do not believe that more precise observations could ever reveal anything more than relative displacements. [quote from Whittaker]]

Whether the ether exists or not matters little -- let us leave that to the metaphysicians; what is essential for us is, that everything happens as if it existed, and that this hypothesis is found to be suitable for the explanation of phenomena. After all, have we any other reason for believing in the existence of material objects? That, too, is only a convenient hypothesis; only, it will never cease to be so, while some day, no doubt, the ether will be thrown aside as useless.

In other words, the aether is just a philosophical construct with no observable consequences. In 1904, he wrote:

The principle of relativity, according to which the laws of physical phenomena must be the same for a stationary observer as for one carried along in a uniform motion of translation, so that we have no means, and can have none, of determining whether or not we are being carried along in such a motion.

... from all these results there must arise an entirely new kind of dynamics, which will be characterized above all by the rule that no velocity can exceed the velocity of light.

It follows from this principle that it must be impossible to observe the aether, if it exists. Einstein's famous 1905 paper on relativity only says this about the aether:

Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the ``light medium,'' suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest. They suggest rather that, as has already been shown to the first order of small quantities, the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good. We will raise this conjecture (the purport of which will hereafter be called the ``Principle of Relativity'') to the status of a postulate, ... The introduction of a ``luminiferous ether'' will prove to be superfluous inasmuch as the view here to be developed will not require an ``absolutely stationary space'' provided with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.

Einstein always claimed that he never read Poincare and his paper did not cite any references. We know that this is a lie because Einstein copied Poincare's ideas and terminology, and because:

The great French mathematician Henri Poincaré enunciated the principle of relativity at least as early as 1902 in his popular book Science and Hypothesis. We know from Einstein's friend Maurice Solovine that the two pounced on Poincaré's book, indeed that it kept them "breathless for weeks on end." It should have. In Science and Hypothesis, Poincaré declares: "1) There is no absolute space, and we can only conceive of relative motion; 2) There is no absolute time. When we say that two periods are equal, the statement has no meaning; 3) Not only have we no direct intuition of the equality of two periods, but we have not even direct intuition of the simultaneity of two events occurring in two different places." These ideas lie at the heart of relativity, and it is hard to imagine that they did not have a profound effect on Einstein's thinking.

Einstein's description of the aether as "superfluous" is almost identical to Poincare's view. Moreover it was Einstein, not Poincare, who reversed himself and later accepted the aether in a 1920 paper:

Therefore I thought in 1905 that in physics one should not speak of the ether at all. This judgment was too radical though as we shall see with the next considerations about the general theory of relativity. It moreover remains, as before, allowed to assume a space-filling medium if one can refer to electromagnetic fields (and thus also for sure matter) as the condition thereof.

Other commenters on Sailor's blog list various other character flaws that are known about Einstein. I am just addressing the origin of special relativity. Credit belongs to Poincare (and Lorentz), not Einstein. Pretty much everything Einstein said on the subject was published earlier by Poincare. You can find more details on Poincare's theory here.